The existing btrfs code does use a lock-free state machine for this, `eb->bflags`, that sort of mirrors regular page flags (hence `UPTODATE`, `DIRTY`, etc.).
But Linux kernel APIs like test_bit(), set_bit(), clear_bit(), test_and_set_bit() etc. only work on one bit at a time. The advantage is they can avoid a CAS loop on many platforms. The disadvantage is you only get atomic transitions for one bit at a time. So the `READING -> UPTODATE` transition is more like
READING -> (READING | UPTODATE) -> UPTODATE
And the `NOT_IN_CACHE -> READING` transition is not fully atomic at all:
if (!(bflags & UPTODATE)) // atomic
// race could happen here
bflags |= READING; // atomic
The whole state machine could be made atomic with CAS, but that would be (slightly) more expensive.
But Linux kernel APIs like test_bit(), set_bit(), clear_bit(), test_and_set_bit() etc. only work on one bit at a time. The advantage is they can avoid a CAS loop on many platforms. The disadvantage is you only get atomic transitions for one bit at a time. So the `READING -> UPTODATE` transition is more like
And the `NOT_IN_CACHE -> READING` transition is not fully atomic at all: The whole state machine could be made atomic with CAS, but that would be (slightly) more expensive.